With the active researches in the fields of genomics or proteomics from the last decade of 20th century up to this day, data has been accumulated concerning types and quantities of genes in various cell groups each regarded as an assembly of cells. Especially, accumulation of data concerning precise comparisons among genome sequences different from species to species, or frequencies of expression of various genes in organs and tissues in one species has reached a level allowing sketchy descriptions of the life process. In the future, various active researches will be made not only for accumulation of data, but also for development of more advanced analysis methods for acquiring data allowing clarification of the life process at a higher level. This clarification of the life process at a higher level is a research scheme called cellomics, and different from the researches using homogeneous cell systems basically prepared by the cloning technique in the past, complex systems each formed with multicellular aggregate each having different functions are treated as objects for research in the field of cellomics.
For promotion of researches in the field of cellomics for comprehensively understanding vital functions as those of a cell assembly, it is necessary to measure expression of genes in discrete cells in the state where functions of each cell structure are preserved, namely in the state where interactions between cells are preserved. To achieve this objective, it is necessary to develop a technique for measuring local expressions of all concerned genes at a level of a cell, which has been impossible in the prior art.
For instance, the genes involved in the circadian rhythm have been identified, and the relation between the cycle and external factors (impetuses) such as insolation has been clarified. The researches as described above are generally carried out using DNA micro-arrays or kinetic PCR which is more quantitative (sometimes called as real time PCR). It is not too much to say, however, that there are few cases, excluding the cases fluctuating with certain cycles such as the circadian rhythm, in which precise data can be obtained from data concerning frequency of gene expressions. The main reason for this is that the relation between the sensitivity and reproducibility has not sufficiently be clarified in the researches using DNA micro-arrays.
To obtain data concerning a frequency of expression of a gene, mRNAs extracted from at least several hundreds of cells are required. By amplifying these mRNAs in the form of cRNAs or cDNAs, the mRNAs can be finally detected in the current situation. If the sensitivity is just insufficient, it is simply required to further amplify the mRNAs in the form of cRNAs or cDNAs, but in the quantitative analysis, sometimes amplifying operations cause errors. Even if it is tried to analyze gene expression (including protein translation) in discrete cells in the tissue, it is impossible to obtain a sufficient quantity of amplification products from a discrete cell with the conventional amplifying operation using, for instance, a DNA micro-array, and even when amplification is forcefully performed to a level allowing for analysis with a DNA micro-array, it is impossible to obtain data reflecting the actual abundance ratio of mRNAs.
Researchers in this field are not satisfied with the knowledge currently available in the fields of genomics and proteomics, and the many researchers are aware of the importance of analysis of a complex system formed with multiple cells at the “cell” level, namely the potential importance of cellomics described above. Further, as an expected application of the cellomics for industrial purposes, many researchers point out the importance of development of the measuring technique at the “cell” level available as an alternative for animal experiments in relation to recent developments of genomic sciences, acceleration of drug developments, and development of pharmaceuticals and chemicals which are safer as compared to those currently available in the market. At present, a vast number of laboratory animals such as guinea pigs or crab-eating macaque are used in experiments for testing the safety or effects of chemical substances such as pharmaceuticals or cosmetics.
However, it has been impossible to overcome the differences between humans and other animals, now the tendency for abolishing the animal experiments has been becoming stronger. When the circumstances as described above are taken into consideration, the preparation, along with its excellent reproducibility, of cell groups each having the minimal functions on the basis of a cell, and of a human cell, and also development of a testing system using the cell groups are conceivably essential to the industries and to realization of safer and more comfortable life of mankind. The cellomics enables researches and development of the technique for realization of the objects as described above.